The recently described class of phosphatidylinositol-anchored membrane surface proteins includes proteins which act as enzymes, receptors for cell-cell interactions, and regulators of complement activation on cell surfaces. Cromer-related antigens, which constitute polymorphisms of decay accelerating factor, comprise the first human erythrocyte blood group known to reside on a phosphatidylinositol-linked protein. Other blood group antigens--including JMH, Holly/Gregory, and Cartwright--also appear to reside on phosphatidylinositol-linked proteins. At least some of these blood group antigens, including Cromer and JMH, are also expressed on leukocytes. This application proposes to study structure and functional effects of blood group antigen-related polymorphisms of decay accelerating factor. In addition, the protein and genetic abnormalities associated with the Inab (Cromer null) and Dr(a-) (Cromer-weak) phenotypes will be investigated. Serum decay accelerating factor will also be investigated, and the abnormal protein expressed by individuals with the Inab phenotype will be characterized both structurally and functionally. Other blood group antigens which reside on and functionally. Other blood group antigens which reside on phosphatidylinositol-linked proteins will be studied, and the specific proteins bearing the antigens will be identified. As the functional roles of these proteins are identified, the effect of polymorphisms on function will also be investigated.